The invention relates to improvements in hinge joints, especially to improvements in hinge joints of the type suitable for use in theseats of motor vehicles. More particularly, the invention relates to improvements in hinge joints of the type wherein two components or leaves are pivotably and radially movably connected to each other by an axially movable coupling member. Still more particularly, the invention relates to improvements in hinge joints of the type wherein a gear transmission or other suitable locking means serves to releasably hold the components of the hinge joint in selected angular positions relative to each other and the locking means comprises an eccentric on the coupling member.
Published European patent application No. A 0048 294 discloses a hinge joint wherein the eccentric of the coupling member constitutes a conical frustum which is surrounded by one of the components, for example, by a spur gear forming an integral part of one of the components. The other component of the hinge joint has two sections which surround, with a certain amount of play, two cylindrical portions of the coupling member. The eccentric is disposed between the cylindrical portions of the coupling member, and the latter is biased axially by a spring so that the eccentric is urged deeper into the complementary opening of the one component. In order to permit a change of inclination of the components relative to each other, the coupling member must be rotated relative to the one component with a force which is sufficient to overcome friction between the parts of the coupling member and the parts of the two components. As such time, a combined force including sliding friction and the restoring force of the spring is weaker than the axial force resulting from radial forces acting upon the coupling member. Thus, the eccentric is shifted axially to thereby account for manufacturing tolerances including radial play between the teeth of gears of the locking means and the clearances between the cylindrical portions of the coupling member and the other component. This ensures that the inclination of the back of the seat which employs one or more hinge joints of the just outlined character can be changed with the exertion of a relatively small force. When the adjustment is completed, the spring moves the coupling member axially to thus eliminate radial play between the gears of the locking means as well as the play between the cylindrical portions of the coupling member and the other component of the hinge joint.
It has been found that the inclination of the back of the seat can be changed in response to the exertion of a relatively small force only if the adjustment of the back is not opposed by an external force, such as the pressure which is exerted by the body of the occupant of the seat. Therefore, the occupant of the seat must lean forwardly to terminate the application of a force against the back of the seat preparatory to and during each adjustment of inclination of the back. The reason for a pronounced increase of resistance to changes of the angular position of the coupling member while the back is acted upon by one or more external forces is that such external force or forces entail the development of radially oriented forces which tend to jam the coupling member so that the latter cannot be readily rotated relative to the one component. Inherent elasticity of parts of the hinge joint also contributes to the need for the exertion of a greater force in order to turn the coupling member while the back of the seat is being acted upon by one or more external forces. When the inclination or orientation of the coupling member relative to the components of the hinge joint deviates from the normal or expected orientation, the area of contact between the coupling member and the components of the hinge joint is reduced considerably; this entails the development of pronounced and highly localized frictional forces which oppose angular movements of the coupling member.